The present invention relates to a method of making a hardened sheet metal article. In addition, the present invention relates to a press mold for carrying out the method.
To ensure clarity, it is necessary to establish the definition of several important terms and expressions that will be used throughout this disclosure. The term “blank” is used here in a generic sense and refers to a flat sheet steel plate as well as to a pre-formed semi-finished product. The term “heat-treating” or “heat treatment” is also used here in a generic sense and includes, i.a., quenching and tempering. The term “press mold” is also used here in a generic sense and includes a progressive die assembly which is capable to perform two or more operations, e.g. pressing, shaping, hardening etc.
German patent publication DE 24 52 486 A1 describes a method of making a hardened sheet metal article from a blank through a press hardening process by heating a blank of hardenable steel to a hardening temperature, placing the blank in a press mold for hot forming to the desired final shape, and hardening the final sheet metal product in the press mold. Since the final sheet metal product remains clamped in the press mold during the cooling process carried out during the hardening operation, the product is accuracy in size.
Through combination of the shaping and hardening and quenching steps in a single tool, hot forming and hardening in the press mold is an cost-effective process.
International patent publication WO 99/07492 discloses a modification of the afore-described press hardening process by collaring marginal areas of a plurality of holes in the press mold. The holes are collared in the press mold before the hardening process. The openings in the sheet metal product serve as through-bores for screw fasteners. These openings, also called in the art as “through-passages”, are utilized as reference holes or seats for accurate alignment of the sheet metal product in subsequent processes. In addition, these openings are also provided as assembly clearance or for reinforcement measure.
The components of this conventional press mold are subjected to great wear, when the openings are shaped. In addition, the collaring operation results only in a limited shaping of the collar. A further drawback is the increased costs compared to the conventional cold forming process.
It would therefore be desirable and advantageous to provide an improved method of making hardened sheet metal articles, which obviates prior art shortcomings and which can be carried out in a more economical and efficient manner.